1. Technical Field
The present invention relates to a liquid jet apparatus and printing apparatus arranged to print predetermined letters and images by emitting microscopic droplets of liquids from a plurality of nozzles to form the microscopic particles (dots) thereof on a printing medium.
2. Related Art
An inkjet printer as one of such printing apparatuses, which is generally low-price and easily provides high quality color prints, has widely been spreading not only to offices but also to general users along with the widespread of personal computers or digital cameras.
Further, in recent inkjet printers, printing in fine tone is required. Tone denotes a state of density of each color included in a pixel expressed by a liquid dot, the size of the liquid dot corresponding to the color density of each pixel is called a tone grade, and the number of the tone grades is called a tone number. The fine tone denotes that the tone number is large. In order for changing the tone grade, it is required to modify a drive pulse to an actuator provided to a liquid jet head. In the case in which a piezoelectric element is used as the actuator, since an amount of displacement (distortion) of the piezoelectric element (a diaphragm, to be precise) becomes large while a voltage value applied to the piezoelectric element becomes large, the tone grade of the liquid dot can be changed using this phenomenon.
Therefore, in JP-A-10-81013, it is arranged that a plurality of drive pulses with different wave heights is combined and joined to generate the drive signal, the drive signal is commonly output to the piezoelectric elements of the nozzles of the same color provided to the liquid jet head, a drive pulse corresponding to the tone grade of the liquid dot to be formed is selected for every nozzle out of the plurality of drive pulses, the selected drive pulses are supplied to the piezoelectric elements of the corresponding nozzles to emit droplets of the liquid different in weight, thereby achieving the required tone grade of the liquid dot.
The method of generating the drive signals (or the drive pulses) is described in FIG. 2 of JP-A-2004-306434. Specifically, the data is retrieved from a memory storing the data of the drive signal, the data is converted into analog data by a D/A converter, and the drive signal is supplied to the liquid jet head through a voltage amplifier and a current amplifier. The circuit configuration of the current amplifier is, as shown in FIG. 3 of JP-A-2004-306434, composed of push-pull connected transistors, and the drive signal is amplified by so called linear drive. However, in the current amplifier with such a configuration, the linear drive of the transistor itself is inefficient, a large-sized transistor is required as a measure against heating of the transistor itself, and moreover, a heat radiation plate for cooling the transistor is required, thus a disadvantage of growth in the circuit size arises, and among others, the size of the heat radiation plate for cooling constitutes a great barrier to design the layout.
In order for overcoming this disadvantage, in the inkjet printer described in JP-A-2005-35062, the drive signals are generated by controlling a reference voltage of a DC/DC converter. In this case, since the DC/DC converter with good efficiency is used, the heat radiation unit for cooling can be eliminated, and further, since a pulse width modulation (PWM) signal is used, a D/A converter can be configured with a simple low-pass filter, thus the circuit size can be made compact.
However, since the DC/DC converter is, in nature, designed to generate a constant voltage, in a head drive device of the inkjet printer described in JP-A-2005-35062 using the DC/DC converter described above, there is caused a problem that the waveform of the drive signal necessary for preferably ejecting an ink droplet from the inkjet head, such as rapid rising or falling waveform can hardly be obtained. Further, in a head drive device of the inkjet printer described in JP-A-2004-306434 for amplifying the current of an actuator drive signal with a push-pull transistor, there is caused a problem that the heat radiation plate for cooking is too large, to substantially complete the layout particularly in a line head printer having a large number of nozzles, namely the actuators.